Vacuum packaging in heat sealable plastic bags is a conventional way of packaging food products such as meat, and cheese. Vacuum packaging typically involves placing the food item in a heat sealable plastic bag having a bag mouth, and then evacuating air from the bag through the bag mouth and collapsing the bag about the contained food item. The bag is then heat sealed in its evacuated condition so the food item becomes encased in a generally air-free environment. Often the bag is a heat shrinkable bag, and after the heat sealing step, is advanced to a hot water or hot air shrink tunnel to induce shrinkage of the bag around the food item.
Vacuum packaging machines of a known type include a vacuum chamber arranged to receive unsealed loaded bags and operable to perform a vacuum sealing operation on the loaded bags. Typically the loaded bags contain products such as meat cuts, arranged in bags formed by a heat-shrinkable film. After feeding a loaded bag to a vacuum chamber and closing the vacuum chamber, the vacuum sealing operation typically includes vacuumization, sealing the mouth of the vacuumized bags, and reintroducing air into the chamber. The chamber is then opened and the vacuum chamber is unloaded. In some applications, the packages may then be conveyed to a heat-shrinking unit to shrink the packaging around the product.
Rotary vacuum packaging machines are known, which include a series of vacuum chambers and chain driven product platens. In operation of the machine the platens move from a loading position, through a vacuum/sealing/venting stage, to an unloading position, and finally back to the loading position. Typical are the packaging machine and machine systems developed by Furukawa Manufacturing Co., Ltd., and disclosed in U.S. Pat. No. 3,958,391 (Kujubu), U.S. Pat. No. 4,580,393 (Furukawa), and U.S. Pat. No. 4,640,081 (Kawaguchi et al.), all incorporated herein by reference in their entirety.
U.S. Pat. No. 7,296,390 (Koke et al.), this patent incorporated herein by reference in its entirety, discloses a vacuum packaging machine including a plurality of vacuum chambers each arranged to receive at least one unsealed loaded bag, and operable to perform an independent vacuum sealing operation, each vacuum chamber having a longitudinal direction defined by a direction of travel of the respective loaded bag through the chamber, each chamber having a heat seal assembly therein which extends transversely to the longitudinal direction. An infeed conveyor is provided for feeding an unsealed loaded bag to a vacuum chamber, such that the trailing (upstream) end of the unsealed loaded bag extends over the heat seal assembly or a part thereof. Each vacuum chamber has a respective bed and independently moveable hood, and the beds can be synchronously vertically moveable. The vacuum packaging machine can be used in combination with a suitable product bagging or wrapping machine.
The system disclosed in U.S. Pat. No. 7,296,390 has been commercialized in New Zealand as the TASVAC™ vacuum packaging system for packaging bagged meat products. In this system, it is beneficial to arrange the trailing (i.e. upstream) edge of each bagged product such that, after heat sealing to close the bag mouth, a bag tail (the portion of the sealed bag located between the trailing edge of the bagged product, and the heat seal) of no more than about two inches (50.8 millimeters) is left on the bag. Bag tails longer than this, e.g. longer than 2 inches (50.8 millimeters), result in use of excessive amounts of thermoplastic materials making up the bag, and also are less aesthetic than bags with shorter bag tails. It is beneficial to have a bag tail less than about 1.5 inches (about 37.6 millimeters), such as one inch (25.4 millimeters) left on the bag. It is also important to have enough bag tail to accommodate irregularly shaped meat cuts, in particular those with a high vertical profile. Put differently, the margin of error for positioning the trailing edge of each product, with respect to the position of the heat assembly, is beneficially a maximum of plus or minus 1.5 inches (about 37.6 millimeters), such as one inch (25.4 millimeters).
Thus, for the TASVAC™ system, in packaging a series of bagged meat products queued on and being advanced by an in-feed conveyor e.g. in a meat processing plant, it is desirable to determine their position vis-à-vis a downstream vacuum chamber, so that each bag can be properly aligned with the heat seal assembly as the loaded bag advances to and inside the vacuum chamber.
Capacitive sensors, positioned upstream of the vacuum chambers on the common in-feed conveyor, have been tested and proven to be unreliable in determining the position of the trailing edge of a product positioned in an unsealed bag, prior to advancing of the bagged product into a given vacuum chamber. This is in part because capacitive sensors rely on the detection of moisture in the meat product being packaged. In a typical meat processing plant, where various meat cuts are being packaged in thermoplastic bags, moisture is commonly present on meat conveyors in the form of water or blood. Use of capacitive sensors therefore sometimes gives a “false positive” indication of the presence of the meat product, such that proper advancement and positioning of each bagged product with respect to the heat seal assembly of a downstream vacuum chamber is difficult and unreliable. In addition, the capacitive sensor has a maximum sensor height range of typically about 40 millimeters.
A product sensor used in the above described apparatus and process should be able to identify product, that is, identify the trailing edge of the product as the product advances along an in-feed conveyor, or is positioned on an in-feed conveyor. The sensor should be able to make such identification for products ranging from thin cuts of veal to thick cuts of fresh red meat to cheese blocks, and should be able to make such identification through clear, printed, and colored bags. Many of these cuts will incorporate odd shapes and profiles. The sensor should also be able to withstand the harsh production environments in meat and cheese packaging plants. The overall cost of the product detection system must be considered as well.
The present invention provides a method for positioning an unsealed loaded bag in a vacuum chamber. The invention enables the sealing of the upstream end of a polymeric bag having therein a product, such as an irregularly shaped meat product, while the loaded bag is in a vacuum chamber, in a manner that provides a bag tail of consistently suitable length.